Exception handling. Aishy Amer. Lecture Outline: Introduction Types or errors or exceptions Conventional error processing Exception handling in C++

Transcription

1 Exception handling Aishy Amer Electrical & Computer Engineering Concordia University Lecture Outline: Introduction Types or errors or exceptions Conventional error processing Exception handling in C++ c A. Amer Exception handling 1

2 Introduction In every code there could be unusual or incorrect states Examples: no memory is allocated; a file is not empty; division by zero;... Many errors will crash a running program Avoid crushes Do not assume that nothing unusual or incorrect will happen when you write your code c A. Amer Exception handling 2

3 Introduction Exception handling: a code for unusual cases, errors, exceptions,... Exception handling: commonly used but not always Exception handling: a C++ technique that transfers control and information from a point in a program to another point in a program called an exception handler An exception handler is associated with a point previously passed by the execution c A. Amer Exception handling 3

4 Introduction Exceptional situation: division by zero what to do? either terminate the program or decide to do something else but what? Exception handling proceeds as follows: 1. Some code (e.g., library software or your code) provides a mechanism that signals the unusual situations Signaling is called throwing an exception 2. A special code of your program deals with the exceptional case Dealing is called handling the exception c A. Amer Exception handling 4

5 Type of errors new returns null pointer no memory allocation was possible Out-of-bounds array subscript Arithmetic overflow Division by zero Invalid function parameter Dereference null pointer... c A. Amer Exception handling 5

6 Conventional error processing Assert Function: terminate if the assert parameter evaluates to false (Evaluates an expression and, when the result is false, prints a diagnostic message and aborts the program) Ignore the error: used in temporary/ad-hoc software projects Abort the program: used in temporary/ad-hoc software projects Flag the error: Software checks flags to handle error c A. Amer Exception handling 6

7 Conventional error processing Report error: send a message to the program user indicating that an error occurred setjump / longjump: allows program to jump out of nested function calls back to a routine that can handle the error Built-in handler routines: For example, set_new_handler c A. Amer Exception handling 7

8 Example without exception handling // Computes the Donuts-Per-Glasses of milk // Assume that we always have some milk void main() { int donuts, milk; double dpg; //Donuts-Per-Glasses cout << "Enter number of donuts and number of glasses of milk:\n"; cin >> donuts; cin >> milk; dpg = donuts/double(milk); cout << donuts << " donuts.\n" << milk << " glasses of milk.\n" << "You have " << dpg << " donuts for each glass of milk.\n"; c A. Amer Exception handling 8

9 Example without exception handling dpg = donuts/double(milk); we may have exceptional situations: 1) no milk left (division-by-zero is not defined) or 2) the number of glasses of milk is negative (a negative amount of milk is meaningless) To handle this exception we need to modify our code c A. Amer Exception handling 9

10 Example with exception handling // Computes donuts per glasses of milk: with Exception Handling void main() { int donuts, milk; double dpg; //Donuts-Per-Glasses cout << "Enter number of donuts and number of glasses of milk:\n"; cin >> donuts; cin >> milk; if (milk <= 0) //prevent division by zero or negative number of glasses // => will handle the exceptional situation cout << donuts << " donuts, and No Milk!\n"; else { dpg = donuts/double(milk); cout << donuts << " donuts.\n" << milk << " glasses of milk.\n" << "You have " << dpg << " donuts for each glass of milk.\n"; c A. Amer Exception handling 10

11 Example using standard exception handling void main() { int donuts, milk; double dpg; //Donuts-Per-Glasses try //**** { cout << "Enter number of donuts and number of glasses of milk:\n"; cin >> donuts; cin >> milk; if (milk <= 0) throw donuts; dpg = donuts/double(milk); cout << donuts << " donuts.\n" << milk << " glasses of milk.\n" << "You have " << dpg << " donuts for each glass of milk.\n"; catch(int e) //**** { cout << e << " donuts, and No Milk!\n"; c A. Amer Exception handling 11

12 Standard exception handling The basic procedure: try-throw-catch try something, if it does not work then throw back the error, a block of code will catch the error and process Each catch has to be in the same scope where its try is try { Try_Code Possibly_Throw_An_Exception More_Code // each try is followed by (at least one) catcher catch( Type catch-block-parameter ) { Handle_Exception c A. Amer Exception handling 12

13 Standard exception handling The thrown value is called an exception The execution of a thrown-statement is called throwing an exception The catch-block is called the exception handler c A. Amer Exception handling 13

14 Exception handling: catch statement The thrown value can be of any type Ex: throw donuts may throw a negative or 0 value to the catcher The catch-block will receive the thrown value (e in the statement catch(int e)) The catch-block is not a function The catch-block parameter has a name for the thrown value that is caught has a type to specifies the type of thrown value to catch c A. Amer Exception handling 14

15 Defining exception classes A throw statement can throw a value of any type Objects too? Common practice: define a class whose objects can carry the information you want to throw to the catch-block With an exception class you can have different types to identify each possible kind of exceptional situation An exception class is a normal class: It is called exception class because it is used to handle exceptions c A. Amer Exception handling 15

16 Defining exception classes: example //Computes donuts per glasses of milk: use an exception class class NoMilk { public: NoMilk(); NoMilk(int how_many); int get_donuts(); private: int count; ; NoMilk::NoMilk() { NoMilk::NoMilk(int how_many){ count = how_many; int NoMilk::get_donuts() { return count; c A. Amer Exception handling 16

17 Defining exception classes: example void main() { int donuts, milk; double dpg; //Donuts-Per-Glasses try { cout << "Enter number of donuts and number of glasses of milk:\n"; cin >> donuts; cin >> milk; if (milk <= 0) throw NoMilk(donuts); dpg = donuts/double(milk); cout << donuts << " donuts.\n" << milk << " glasses of milk.\n" << "You have " << dpg << " donuts for each glass of milk.\n"; catch(nomilk e) { cout << e.get_donuts() << " donuts, and No Milk!\n"; c A. Amer Exception handling 17

18 Defining exception classes: comments At throw NoMilk(donuts); the constructor of the class NoMilk is called This constructor takes one int argument and creates an object of the class NoMilk That object is then "thrown" c A. Amer Exception handling 18

19 Handling multiple exceptions The thrown value can have different types the catch-block should handle the different types When catching multiple exceptions, the order of the catch-blocks can be important the catch-block will be tried in the order that they appear in the program thus, the first one that matches the type of the exception thrown is the one that is executed c A. Amer Exception handling 19

20 Handling multiple exceptions try {... if(condition A) throw "This is a char* exception"; if(condition B) throw 5; if(condition C) throw DivideByZeroError(); //DivideByZeroError is a? catch( char* Estring ) { cout << Estring << endl; catch( int errnum ) { cout << errnum << endl; catch( DivideByZeroError DBZE ) { cout << DBZE.what() << endl; If no catch block matches the object thrown, then terminate() is called defaulting in a call to abort!!! c A. Amer Exception handling 20

21 Special catch-block A special kind of catch-block that will catch a thrown value of any type: catch (...) // The three dots must be placed { <handle unknown thrown types> // Example: cout<< "classyy::funcxx(): unknown error\n"; This is the most general case, so this catch should be included at the end after all other cases c A. Amer Exception handling 21

22 Nested try blocks Nested try blocks: The closest matching catch block is executed try { try { throw 5; //**** catch(int e) { cout << "Inner catch: " << e << endl; catch(...) { cout << "nested try: unknown error\n"; throw 7; //**** // end of outer try catch(int e) // for each catch there has to be a throw! { cout << "Outer catch: "<< e<< endl; catch(...) { cout<< "outer: unknown error\n"; c A. Amer Exception handling 22

23 Re-throws The exception cannot be handled by the catch block? re-thrown it try { if(t==10) throw 5; catch( int errnum ) { // each catch has to be in the same scope where its try is cout<<"inner catch\n"; try { if (errnum == 5) { throw; //Re-throw else { cout << errnum; // end inner try catch(int errnum) { cout << "Outer Catch Block: "<< errnum << endl; catch(...) { cout << "Outer Catch Block: unknown error\n"; //end of outer catch c A. Amer Exception handling 23

24 Exception handling: classes Example: Compute fraction (numerator/denominator) Use an inverse function Define an exception class MSG that handles the various exceptional situations Zero denominator Negative denominator Numerator zero (?)... c A. Amer Exception handling 24

25 Exception handling: classes //Listing 18.7 Throwing class objects rather than built-in values class MSG { //Messages mainly corresponding to exceptions private: static char* data [6]; // static: one instance for all obj. public: static char* msg(int n) { // public static method if (n<1 n > 5) // check index validity return data[0]; else return data[n]; // return valid string ; c A. Amer Exception handling 25

26 Exception handling: classes // Before use: initialize the static data: Array of strings char* MSG::data [] = { "\nbad argument to msg()\n", "\nzero denominator is not allowed\n\n", "\nnegative denominator: ", "Enter numerator and positive\n", "denominator (any letter to quit): ", "Value of the fraction: " ; c A. Amer Exception handling 26

27 Exception handling: classes class ZeroDenom { // Define ZeroDenom exception private: char *msg; // data to be carried to error handler public: ZeroDenom () { // it is called by the throw statement msg = MSG::msg(1); // the message is the second element of data[] void print () const // it is called by the catch block { cout << msg; ; c A. Amer Exception handling 27

28 Exception handling: classes class NegativeDenom { // Define NegativeDenom exception private: long val; // private data for exception info char* msg; public: NegativeDenom(long value) // conversion constructor { val = value; msg = MSG::msg(2); char* getmsg() const { return msg; long getval() const { return val; ; // public methods to access data c A. Amer Exception handling 28

29 Exception handling: classes inline void inverse(long value, double& answer) { answer = (value)? 1.0/value : DBL_MAX; if (answer==dbl_max) throw ZeroDenom(); if (value < 0) throw NegativeDenom(value); inline void fraction (long numer, long denom, double& result) { inverse(denom, result); // result = 1.0 / denom result = numer * result; // result = numer/denom c A. Amer Exception handling 29

30 Exception handling: classes //... First initialize the static class data: Array of strings int main() { long numer, denom; double ans; // numerator, denominator cout << MSG::msg(3) << MSG::msg(4); // prompt user for data cin >> numer >> denom; // enter data try { fraction(numer,denom,ans); // compute answer cout << MSG::msg(5) << ans <<"\n\n"; // valid answer catch (const ZeroDenom& zd) // zero denominator { zd.print(); catch (const NegativeDenom &nd) // negative value { cout << nd.getmsg() << nd.getval() << "\n\n"; catch(...) { cout<< "Unkown Throw\n" return 0; c A. Amer Exception handling 30

31 Exception handling: Why Many errors, such as divide by zero, will crash a running program Exception handling allows the program to recognize the error, deal with it (either recover or "safely" exit), and return to program execution It provides an "elegant" way to handle and recover from errors "Improves program readability and modifiability" it permits easy maintenance of existing code c A. Amer Exception handling 31

32 Exception handling: When In situations when the system can recover from the error Ex.: out-of-bounds array subscript In situations where the system cannot recover Valued information is saved and the system is shut down gracefully Example: system runs out of RAM from a memory leak Example: breaking a communication network Use exception handling only when it is not possible to handle the error locally c A. Amer Exception handling 32

33 Exception handling: Why not? Exceptions should be used in exceptional cases only Exception handling makes programs larger, slower, and harder (often, much harder) to understand Exception handling is less efficient than conventional handling takes more time to process returns the program in a different state when the processing is finished c A. Amer Exception handling 33